(1) Field of the Invention
This invention relates generally to DC-to-DC converters and relates more specifically to non-inverted buck-boost switching regulators and control methods thereof.
(2) Description of the Prior Art
The buck-boost converter is a type of DC-DC converter that has an output voltage magnitude that is either greater than or less than the input voltage magnitude. It is a switch mode power supply with a similar circuit topology to the boost converter and the buck converter. The output voltage is adjustable based on the duty cycle of the switching transistor.
FIG. 1 prior art shows a non-inverted buck-boost switching regulator topology. Switches S1-S4 are usually realized by transistors.                while in the On-state, S1 and S3 are closed (on) and the input voltage source is connected to the inductor L. This results in accumulating energy in L. In this stage, the capacitor Cout supplies energy to the output load.        while in the Off-state, S2 and S4 are closed (on) and the inductor L is connected to the output load and capacitor Cout, so energy is transferred from L to Cout and Load.        
Compared to the buck and boost converters, the characteristics of the buck-boost converter are mainly:                The output voltage can vary continuously from 0 to −∞(for an ideal converter). The output voltage ranges for a buck and a boost converter are respectively 0 to Vi and Vi to ∞.        
However, the lower conversion efficiency is the downside of the buck-boost converter. All four switches have to work all the time, whatever the input voltage is lower than the output voltage or higher than the output voltage. This increases the driving loss of switches and lowers the efficiency. In addition to this, the inductor current gets higher than in the buck or boost operation mode and increases the conduction loss of switches and the efficiency is further reduced.
U.S. patent (U.S. Pat. No. 6,166,527 to Dwelley et al., application Ser. No. 09/536,266) discloses a different control method to alleviate the above efficiency issue. In this method disclosed the buck-boost converter works in three different modes, namely in buck, boost or buck-boost mode. In buck mode S4 is always on and S3 is always off. This creates a buck topology in the buck-boost circuit and switches S1 and S2 are switching the same as a buck converter, when the input voltage is higher than the output voltage. In boost mode switch S1 is always on and switch S2 is always off, hence creating a boost topology. Switches S3 and S4 are switching and regulate the output voltage, when the input voltage is lower than the output voltage.
In buck-boost mode, all four switches are switching but use different PWM signals for the pair of S1 and S2 switches and the S3 and S4 pair in order to reduce the required inductor current. This control method improves the efficiency compared to the classical control method, but in the buck-boost mode all four switches have to work all the time at full frequency, so the driving loss is still high and reduces the efficiency in the buck-boost mode.
Another way to improve the efficiency of the buck-boost converter is, to remove the buck-boost operation from above invention. The converter only has buck mode and boost mode. When the input voltage is close to the output voltage, switches S1 and S4 are ON most of time, and switching intermittently occurs either in buck mode or boost mode. The drawback of this control method is that the output voltage regulation is poor, and frequency is not constant when input voltage and output voltage is close, and it may cause unwanted noise problems to the system. This method is called ‘pulse skipping architecture.
It is a challenge for engineers to design high-efficient buck-boost converters, especially in conditions when the input voltage is close to the output voltage.
There are known patents or patent publications dealing with buck-boost converters.
U.S. Patent Publication (US 2010/0148740 to Saitoh) discloses a buck-boost regulator providing a stable, high-speed, high-efficiency constant voltage without a complicated, large-scale, high-cost phase compensation circuit over a wide range of operating conditions. This voltage buck-boost switching regulator consists of a pair of voltage reducing transistors, a pair of voltage boosting transistors, inductance coil, output capacitor, and controller. The controller has the following parts for performing PWM control of constant voltage for voltage reducing transistors and voltage boosting transistors: an output voltage feedback circuit, an inductor current sense circuit, a variable saw-tooth wave signal generator, switching controllers, and a voltage boosting driver.
U.S. Patent Publication (US 2009/0295343 to Chiu) discloses a buck-boost switching regulator, comprising: (1) a first loop including: a first and a second switch electrically connected with each other, the first switch having an end electrically connected with an input voltage, and the second switch having an end electrically connected with ground; and a first control circuit controlling the operation of the first and the second switch; (2) a second loop including: a third and a fourth switch electrically connected with each other, the third switch having an end electrically connected with ground, and the fourth switch having an end electrically connected with an output voltage; and a second control circuit controlling the operation of the third and the fourth switch; and (3) an inductor electrically connected between a node between the first and the second switch, and a node between the third and the fourth switch.
U.S. patent (U.S. Pat. No. 7,737,668 to Oswald et al.) proposes a buck-boost switching regulator which includes a first switch, a first diode, an inductor, a second switch, a second diode, and a controller for controlling the first switch and the second switch, the controller being configured to receive a current signal indicative of a inductor current flowing in the inductor, and generating a signal indicative of an average current flowing in the inductor, the average current being utilized to control the first switch and the second switch, wherein the controller includes a first compensator circuit for outputting a voltage error signal, a second compensator circuit for outputting a current error signal and a modulator circuit to output a first control signal to control the first switch and a second control switch to control the second switch.